This invention relates to ultrasonic systems and, more particularly, to apparatus for imaging sections of a body by transmitting ultrasonic energy into the body and determining the characteristics of the ultrasonic energy reflected therefrom, the apparatus having the capability of detecting the velocity of motion of material within the body.
In recent years ultrasonic techniques have become more prevalent in clinical diagnosis. Such techniques have been utilized for some time in the field of obstetrics, neurology and cardiology, and are becoming increasingly important in the visualization of a number of different body portions, for example, the scanning of breasts to detect tumors.
Various fundamental factors have given rise to the increased use of ultrasonic techniques. Ultrasound differs from other forms of radiation in its interaction with living systems in that it has the nature of a mechanical wave. Accordingly, information is available from its use which is of a different nature than that obtained by other methods and it is found to be complementary to other diagnostic methods, such as those employing X-rays. Also, the risk of tissue damage using ultrasound appears to be much less than the apparent risk associated with ionizing radiations such as X-rays.
The majority of diagnostic techniques using ultrasound are based on the pulse-echo method wherein pulses of ultrasonic energy are periodically generated by a suitable piezoelectric transducer such as a lead zirconate-titanate ceramic. Each short pulse of ultrasonic energy is focused to a narrow beam which is transmitted into the patient's body wherein it eventually encounters interfaces between various different structures of the body. When there is a characteristic impedance mismatch at an interface, a portion of the ultrasonic energy is reflected at the boundary back toward the transducer. After generation of the pulse, the transducer operates in a "listening" mode wherein it converts received reflected energy or "echoes" from the body back into electrical signals. The time of arrival of these echoes depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface.
There are various ways in which the information in the received echoes can be usefully presented. In one common technique, the electrical signal representative of detected echoes are amplified and applied to the vertical deflection plates of a cathode ray display. The output of a time-base generator is applied to the horizontal deflection plates. Continuous repetition of the pulse/echo process in sychronism with the time-base signals produces a continuous display, called an "A-scan", in which time is proportional to range, and deflections in the vertical direction represent the presence of interfaces. The height of these vertical deflections is representative of echo strength.
Another common form of display is the so-called "B-scan" wherein the echo information is of a form more similar to conventional television display; i.e., the received echo signals are utilized to modulate the brightness of the display at each point scanned. This type of display is found especially useful when the ultrasonic energy is scanned transverse the body so that individual "ranging" information yields individual scan lines on the display, and successive transverse positions are utilized to obtain successive scan lines on the display. The two-dimensional B-scan technique yields a cross-sectional picture in the plane of the scan, and the resultant display can be viewed directly or recorded photographically or on magnetic tape.
In addition to its application for imaging internal features of a body, as described, ultrasound has also been employed for measuring the velocity of motion of material within a body by employing established Doppler detection techniques. Typically, an ultrasound beam is directed such that a component thereof is in the direction (or opposed to the direction) of flow of blood moving in a body. As is well known, the returning ultrasound echoes will have a Doppler shifted frequency component that is a function of the blood velocity.
It has been recognized that it would be highly desirable to have a single equipment which has a capability of both ultrasonic imaging and ultrasonic blood velocity measurement. With such an integrated equipment the user could, for example, locate a specific position in a blood vessel using a displayed image, and also obtain a measure of blood velocity (or, once geometry is known, flow rate) at the specific position. In one recently developed scheme, an ultrasonic Doppler flow meter subsystem is used in conjunction with a two dimensional B-scan ultrasonic imaging system. The imaging system utilizes an array of one or more transducers that is directed approximately normal to the patient's skin, as is conventional. To obtain Doppler flow measurements, at least one separate transducer is mounted on the same assembly as the B-scan system. This separate transducer is oriented at an angle with respect to the perpendicular to the skin, so that the beam emitted therefrom has a component in the direction of flow of blood vessels flowing beneath the skin. By processing the output of the separate transducer, a range-gated Doppler signal can be obtained and presented (e.g. an audio or video presentation) which represents the flow velocity of blood at the range-gated position at which the separate transducer is aimed. It would be desirable to have the B-scan image presented simultaneously with the Doppler information from the same region in the body. However, interference can occur between the two separate ultrasound subsystems if they are operated together. Accordingly, in order to obtain simultaneous presentation of information without excessive interference, one prior art scheme stores and repetitively presents the B-scan image while the separate transducer is operative to obtain Doppler flow information for "simultaneous" presentation. However, in addition to the complexity and cost of such a scheme, problems can arise from movements of the scanner head during the time that the Doppler subsystem is active, since such movements result in "misalignment" of the Doppler information with the stored B-scan information that is being presented.
It is among the objects of this invention to provide an improved Doppler detection system and an advantageous combination of an ultrasonic imaging subsystem and a Doppler flow meter subsystem that overcomes the type of problems set forth and provides a more efficient and less expensive technique for simultaneous presentation of useful information.